<?php
function larger ($x, $y)
{
  if (!isset($x)||!isset($y))
    return false;
  else if ($x>=$y)
    return $x;
  else
    return $y;
}

$a = 1; $b = 2.5; $c = 1.9;
echo larger($a, $b).'<br />';
echo larger($c, $a).'<br />';
echo larger($d, $a).'<br />';

?>